1,285 research outputs found
Quantum Nondemolition Measurement of a Kicked Qubit
We propose a quantum nondemolition measurement using a kicked two-state
system (qubit). By tuning the waiting time between kicks to be the qubit
oscillation period, the kicking apparatus performs a nondemolition measurement.
While dephasing is unavoidable, the nondemolition measurement can (1) slow
relaxation of diagonal density matrix elements, (2) avoid detector back-action,
and (3) allow for a large signal-to-noise ratio. Deviations from the ideal
behavior are studied by allowing for detuning of the waiting time, as well as
finite-time, noisy pulses. The scheme is illustrated with a double-dot qubit
measured by a gate-pulsed quantum point contact.Comment: 7 pages, 1 figur
Persistent current magnification in a double quantum-ring system
The electronic transport in a system of two quantum rings side-coupled to a
quantum wire is studied via a single-band tunneling tight-binding Hamiltonian.
We derived analytical expressions for the conductance, density of states and
the persistent current when the rings are threaded by magnetic fluxes. We found
a clear manifestation of the presence of bound states in each one of those
physical quantities when either the flux difference or the sum of the fluxes
are zero or integer multiples of a quantum of flux. These bound states play an
important role in the magnification of the persistent current in the rings. We
also found that the persistent current keeps a large amplitude even for strong
ring-wire coupling.Comment: 15 pages, 10 figures. Submitted to PR
Mesoscopic Capacitance Oscillations
We examine oscillations as a function of Fermi energy in the capacitance of a
mesoscopic cavity connected via a single quantum channel to a metallic contact
and capacitively coupled to a back gate. The oscillations depend on the
distribution of single levels in the cavity, the interaction strength and the
transmission probability through the quantum channel. We use a Hartree-Fock
approach to exclude self-interaction. The sample specific capacitance
oscillations are in marked contrast to the charge relaxation resistance, which
together with the capacitance defines the RC-time, and which for spin polarized
electrons is quantized at half a resistance quantum. Both the capacitance
oscillations and the quantized charge relaxation resistance are seen in a
strikingly clear manner in a recent experiment.Comment: 9 pages, 2 figure
Low frequency admittance of a quantum point contact
We present a current and charge conserving theory for the low frequency
admittance of a quantum point contact. We derive expressions for the
electrochemical capacitance and the displacement current. The latter is
determined by the {\em emittance} which equals the capacitance only in the
limit of vanishing transmission. With the opening of channels the capacitance
and the emittance decrease in a step-like manner in synchronism with the
conductance steps. For vanishing reflection, the capacitance vanishes and the
emittance is negative.Comment: 11 pages, revtex file, 2 ps figure
Electron transport in an open mesoscopic metallic ring
We study electron transport in a normal-metal ring modeled by the tight
binding lattice Hamiltonian, coupled to two electron reservoirs. First,
Buttiker's model of incorporating inelastic scattering, hence decoherence and
dissipation, has been extended by connecting each site of the open ring to
one-dimensional leads for uniform dephasing in the ring threaded by magnetic
flux. We show with this extension conductance remains symmetric under flux
reversal, and Aharonov-Bohm oscillations with changing magnetic flux reduce to
zero as a function of the decoherence parameter, thus indicating dephasing in
the ring. This extension enables us to find local chemical potential profiles
of the ring sites with changing magnetic flux and the decoherence parameter
analogously to the four probe measurement. The local electrochemical potential
oscillates in the ring sites because of quantum-interference effects. It
predicts that measured four-point resistance also fluctuates and even can be
negative. Then we point out the role of the closed ring's electronic
eigenstates in the persistent current around Fano antiresonances of an
asymmetric open ring for both ideal leads and tunnel barriers. Determining the
real eigenvalues of the non-Hermitian effective Hamiltonian of the ring, we
show that there exist discrete bound states in the continuum of scattering
states for the asymmetric ring even in the absence of magnetic flux. Our
approach involves quantum Langevin equations and non-equilibrium Green's
functions.Comment: 19 pages, 6 figure
Parity detection and entanglement with a Mach-Zehnder interferometer
A parity meter projects the state of two qubits onto two subspaces with
different parities, the states in each parity class being indistinguishable. It
has application in quantum information for its entanglement properties. In our
work we consider the electronic Mach-Zehnder interferometer (MZI) coupled
capacitively to two double quantum dots (DQDs), one on each arm of the MZI.
These charge qubits couple linearly to the charge in the arms of the MZI. A key
advantage of an MZI is that the qubits are well separated in distance so that
mutual interaction between them is avoided. Assuming equal coupling between
both DQDs and the arms and the same bias for each DQD, this setup usually
detects three different currents, one for the odd states and two for each even
state. Controlling the magnetic flux of the MZI, we can operate the MZI as a
parity meter: only two currents are measured at the output, one for each parity
class. In this configuration, the MZI acts as an ideal detector, its Heisenberg
efficiency being maximal. For a class of initial states, the initially
unentangled DQDs become entangled through the parity measurement process with
probability one.Comment: 9 pages, 2 figure
Full counting statistics for voltage and dephasing probes
We present a stochastic path integral method to calculate the full counting
statistics of conductors with energy conserving dephasing probes and
dissipative voltage probes. The approach is explained for the experimentally
important case of a Mach-Zehnder interferometer, but is easily generalized to
more complicated setups. For all geometries where dephasing may be modeled by a
single one-channel dephasing probe we prove that our method yields the same
full counting statistics as phase averaging of the cumulant generating
function.Comment: 4 pages, 2 figure
Charge Fluctuations in Quantum Point Contacts and Chaotic Cavities in the Presence of Transport
We analyze the frequency-dependent current fluctuations induced into a gate
near a quantum point contact or a quantum chaotic cavity. We use a current and
charge conserving, effective scattering approach in which interactions are
treated in random phase approximation. The current fluctuations measured at a
nearby gate, coupled capacitively to the conductor, are determined by the
screened charge fluctuations of the conductor. Both the equilibrium and the
non-equilibrium current noise at the gate can be expressed with the help of
resistances which are related to the charge dynamics on the conductor. We
evaluate these resistances for a point contact and determine their
distributions for an ensemble of chaotic cavities. For a quantum point contact
these resistances exhibit pronounced oscillations with the opening of new
channels. For a chaotic cavity coupled to one channel point contacts the charge
relaxation resistance shows a broad distribution between 1/4 and 1/2 of a
resistance quantum. The non-equilibrium resistance exhibits a broad
distribution between zero and 1/4 of a resistance quantum.Comment: 9 pages, two-column Revtex, 6 figures include
Time dependence of evanescent quantum waves
The time dependence of quantum evanescent waves generated by a point source
with an infinite or a limited frequency band is analyzed. The evanescent wave
is characterized by a forerunner (transient) related to the precise way the
source is switched on. It is followed by an asymptotic, monochromatic wave
which at long times reveals the oscillation frequency of the source. For a
source with a sharp onset the forerunner is exponentially larger than the
monochromatic solution and a transition from the transient regime to the
asymtotic regime occurs only at asymptotically large times. In this case, the
traversal time for tunneling plays already a role only in the transient regime.
To enhance the monochromatic solution compared to the forerunner we investigate
(a) frequency band limited sources and (b) the short time Fourier analysis (the
spectrogram) corresponding to a detector which is frequency band limited.
Neither of these two methods leads to a precise determination of the traversal
time. However, if they are limited to determine the traversal time only with a
precision of the traversal time itself both methods are successful: In this
case the transient behavior of the evanescent waves is at a time of the order
of the traversal time followed by a monochromatic wave which reveals the
frequency of the source.Comment: 16 text pages and 9 postscript figure
Time-Dependent Current Partition in Mesoscopic Conductors
The currents at the terminals of a mesoscopic conductor are evaluated in the
presence of slowly oscillating potentials applied to the contacts of the
sample. The need to find a charge and current conserving solution to this
dynamic current partition problem is emphasized. We present results for the
electro-chemical admittance describing the long range Coulomb interaction in a
Hartree approach. For multiply connected samples we discuss the symmetry of the
admittance under reversal of an Aharonov-Bohm flux.Comment: 22 pages, 3 figures upon request, IBM RC 1971
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